A spectrum analyzer basically comprises a frequency converter circuit formed of a mixer which is coupled to receive an input signal under measurement and a local oscillator circuit whose output signal is also input to the mixer, together with means for sweeping the output signal frequency of the local oscillator circuit, an intermediate frequency amplifier circuit including a band-pass filter whose bandwidth determines the resolution bandwidth of the spectrum analyzer, an envelope detector circuit for detecting the output signal from the intermediate frequency amplifier circuit, and display means to display the detector output signal as the frequency spectrum of the input signal under measurement (or, more generally, a portion of that frequency spectrum). In addition, an input attenuator is provided coupled between the source of the input signal under measurement and the input of the mixer, to enable adjustment of the level of input signal applied to the mixer. The resolution bandwidth can be preset to a desired value by the user, together with the reference level. As used hereinafter, the term "reference level" denotes the level of input signal applied to the spectrum analyzer which will, with the currently set values of input attenuation and intermediate frequency amplification, be displayed at a predetermined reference position on the display device of the spectrum analyzer, for example at a specific signal amplitude scale graduation on a CRT display. The reference level is determined by the overall amount of gain from the point of input to the spectrum analyzer to the output from the intermediate frequency amplifier circuit, and can be altered by varying the amount of input attenuation or the amount of intermediate frequency amplification, or a combination of both of these factors.
For a given value of reference level, the average noise level of the output signal from the intermediate frequency amplifier circuit varies with the resolution bandwidth, i.e., as the resolution bandwidth is increased the average noise level increases and as the resolution bandwidth is decreased the average noise level decreases. The average noise level is also relatively a parameter of the input signal level applied to the mixer, that is to say as the input signal level is increased (for a given value of reference level) the average noise level is decreased relatively, and vice versa. On the other hand, as the input signal level applied to the mixer is increased, the amount of distortion, principally harmonic distortion, which is generated in the mixer will increase, while this distortion is decreased as the input signal level to the mixer is decreased. Thus, for any particular combination of reference level and resolution bandwidth, there is a corresponding combination of average noise level and harmonic distortion (measured relative to the input signal under measurement, e.g., each measured as a proportion of the displayed signal amplitude) which will be optimum, i.e., such that the average noise level and harmonic distortion components of the displayed spectrum are substantially equal. With these values of average noise level and harmonic distortion, the dynamic range of the spectrum analyzer is maximized, and therefore it is desirable to establish such values of average noise level and harmonic distortion as far as possible.
However with prior spectrum analyzers, no means for optimizing these parameters is provided. Instead, the level of input signal applied to the mixer is held to an approximately constant value by adjustment (usually performed manually) of the degree of input attenuation. That is to say, the reference level is set to be approximately equal to the level of the input signal under measurement. In some cases it may be possible for the user to vary the degree of input attenuation while holding a fixed reference level value, i.e., when a relatively narrow resolution bandwidth value has been set, then in order to increase the dynamic range of the spectrum analyzer, the user can increase the amount of input attenuation applied to the input signal under measurement so as to reduce the amount of harmonic distortion generated by the mixer, and can compensate for this increased attenuation be increasing the amount of intermediate frequency amplification such as to restore the original reference level value. Conversely, if a relatively broad resolution bandwidth value has been set, then the user can maximize the dynamic range by decreasing the amount of attenuation applied to the input signal under measurement so as to reduce the average noise level, and can compensate for this decreased input attenuation by increasing the amount of intermediate frequency amplification. However it will be apparent that such a procedure will require a considerable degree of experience in order to be effectively utilized, and that there is a considerable danger of measurement errors occurring.
There is therefore a requirement for a spectrum analyzer which is provided with means for automatically maintaining the level of attenuation applied to the input signal under measurement, taking into consideration the currently set values of reference level and resolution bandwidth, such as to establish optimum values of average noise level and harmonic distortion at all times.